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djm pushed a commit to branch master
in repository openssh.

commit 90ccc5918ea505bf156c31148b6b59a1bf5d6dc6
Author: djm@openbsd.org <djm@openbsd.org>
Date: Sun Sep 10 03:25:53 2023 +0000

upstream: randomise keystroke obfuscation intervals and average

interval rate. ok dtucker@

OpenBSD-Commit-ID: 05f61d051ab418fcfc4857ff306e420037502382
---
clientloop.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 46 insertions(+), 6 deletions(-)

diff --git a/clientloop.c b/clientloop.c
index 93bd9343..b3a3b36e 100644
--- a/clientloop.c
+++ b/clientloop.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: clientloop.c,v 1.396 2023/09/04 00:08:14 djm Exp $ */
+/* $OpenBSD: clientloop.c,v 1.397 2023/09/10 03:25:53 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -118,6 +118,9 @@
/* Permitted RSA signature algorithms for UpdateHostkeys proofs */
#define HOSTKEY_PROOF_RSA_ALGS "rsa-sha2-512,rsa-sha2-256"

+/* Uncertainty (in percent) of keystroke timing intervals */
+#define SSH_KEYSTROKE_TIMING_FUZZ 10
+
/* import options */
extern Options options;

@@ -528,6 +531,43 @@ send_chaff(struct ssh *ssh)
return 1;
}

+/* Sets the next interval to send a keystroke or chaff packet */
+static void
+set_next_interval(const struct timespec *now, struct timespec *next_interval,
+ u_int interval_ms, int starting)
+{
+ struct timespec tmp;
+ long long interval_ns, fuzz_ns;
+ static long long rate_fuzz;
+
+ interval_ns = interval_ms * (1000LL * 1000);
+ fuzz_ns = (interval_ns * SSH_KEYSTROKE_TIMING_FUZZ) / 100;
+ /* Center fuzz around requested interval */
+ if (fuzz_ns > INT_MAX)
+ fuzz_ns = INT_MAX;
+ if (fuzz_ns > interval_ns) {
+ /* Shouldn't happen */
+ fatal_f("internal error: fuzz %u%% %lldns > interval %lldns",
+ SSH_KEYSTROKE_TIMING_FUZZ, fuzz_ns, interval_ns);
+ }
+ /*
+ * Randomise the keystroke/chaff intervals in two ways:
+ * 1. Each interval has some random jitter applied to make the
+ * interval-to-interval time unpredictable.
+ * 2. The overall interval rate is also randomly perturbed for each
+ * each chaffing session to make the average rate unpredictable.
+ */
+ if (starting)
+ rate_fuzz = arc4random_uniform(fuzz_ns);
+ interval_ns -= fuzz_ns;
+ interval_ns += arc4random_uniform(fuzz_ns) + rate_fuzz;
+
+ tmp.tv_sec = interval_ns / (1000 * 1000 * 1000);
+ tmp.tv_nsec = interval_ns % (1000 * 1000 * 1000);
+
+ timespecadd(now, &tmp, next_interval);
+}
+
/*
* Performs keystroke timing obfuscation. Returns non-zero if the
* output fd should be polled.
@@ -591,12 +631,12 @@ obfuscate_keystroke_timing(struct ssh *ssh, struct timespec *timeout,
*/
if (!active && ssh_packet_interactive_data_to_write(ssh) &&
channel_did_enqueue && ssh_packet_have_data_to_write(ssh)) {
- debug3_f("starting: interval %d",
+ debug3_f("starting: interval ~%dms",
options.obscure_keystroke_timing_interval);
just_started = had_keystroke = active = 1;
nchaff = 0;
- ms_to_timespec(&tmp, options.obscure_keystroke_timing_interval);
- timespecadd(&now, &tmp, &next_interval);
+ set_next_interval(&now, &next_interval,
+ options.obscure_keystroke_timing_interval, 1);
}

/* Don't hold off if obfuscation inactive */
@@ -629,8 +669,8 @@ obfuscate_keystroke_timing(struct ssh *ssh, struct timespec *timeout,
n = (n < 0) ? 1 : n + 1;

/* Advance to the next interval */
- ms_to_timespec(&tmp, options.obscure_keystroke_timing_interval * n);
- timespecadd(&now, &tmp, &next_interval);
+ set_next_interval(&now, &next_interval,
+ options.obscure_keystroke_timing_interval * n, 0);
return 1;
}


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